To build lighting systems for illumination and projection, there are significant advantages to being able to tailor the shape of the light source, since the shape of the light source and the optical components of the system provide the means to precisely shape the resulting light beam. The shape of the resulting light beam is an important aspect of the lighting system, especially in the creation of solid-state headlamps for the automotive industry, as disclosed in United States Published Patent Applications Nos. 2005/088853, entitled Vehicle Lamp, published Apr. 28, 2005 to Yatsuda et al; and 2005/041434, entitled Light Source and Vehicle Lamp, published Feb. 24, 2005 to Yasushi Yatsuda et al. The principle of operation is to construct an arrangement of light-source elements positioned in such a manner as to form an emission shape and a brightness distribution that can create a light distribution pattern when combined with suitable optics.
Unfortunately, conventional shaped light emitting devices must be constructed from a number of individual light emitting elements, such as LEDs, which typically cannot be constructed with an area greater than about four mm2 due to inherent limitations in compound semiconductor processing technologies, e.g. a lattice mismatch between substrate and active layers. Moreover, the individual light emitting elements typically cannot be positioned within five millimeters of each other, because of the need to provide physical mounting, optical coupling and electrical interconnection for each of the individual elements. Accordingly, the emissive shapes constructed do not provide a contiguous illuminated area, and have inherent limitations on the available brightness per unit area. Furthermore, the refinement or smoothness of the shape is limited by the granularity of the individual lighting elements, and the light emitting elements cannot be made smaller than a certain size because of the physical constraints in their mounting and interconnection.
Recent research into the nature of electrical conduction and light emission from nano-particles formed in wide bandgap semiconductor materials or insulating dielectrics has been conducted in an effort to increase the conductivity of the wide bandgap dielectric semiconductor materials, which exhibit very little conductivity, through the formation of nano-particles within the insulating material. With the application of a suitable electric field, current can be made to flow through the tunneling process, which can transfer energy efficiently from the applied electric field to the nano-particles and store that energy in the form of excitons through the impact ionization process in the silicon nano-particles. The excitons can radiatively recombine releasing a photon, whose energy is determined by the size of the nano-particles in the wider bandgap material or the nano-particles can transfer the energy to a rare earth dopant, which will emit a photon at a characteristic wavelength. A wide bandgap dielectric layer with nano-particles constitutes an optically active layer including a concentration of luminescent centers. Several materials can be used as the wide bandgap semiconductor or dielectric material including GaN, silicon nitride, and silicon dioxide. The luminescent centers can be formed from a wide variety and combination of compatible materials including silicon, carbon, germanium, and various rare earths.
For technical and economic reasons, Silicon Rich Silicon Oxide (SRSO) films are being developed for the purposes of studying the efficient generation of light from silicon based materials. The SRSO films consist of silicon dioxide in which there is excess silicon and possibly the incorporation of rare earths into the oxide. The amount of excess silicon will determine the electrical properties of the film, specifically the bulk conductivity and permittivity. With the excess silicon in the oxide, the film is annealed at a high temperature, which results in the excess silicon coalescing into tiny silicon nano-particles, e.g. nanocrystals, dispersed through a bulk oxide film host matrix. The size and distribution of the silicon nano-particles can be influenced by the excess silicon originally incorporated at deposition and the annealing conditions.
Optically active layers formed using semiconductor nano-particles embedded within a wider bandgap semiconductor or dielectric have been demonstrated in U.S. Pat. No. 7,081,664, entitled: “Doped Semiconductor Powder and Preparation Thereof”, issued Jul. 25, 2006 in the name of Hill; and U.S. Pat. No. 7,122,842, entitled Solid State White Light Emitter and Display Using Same, issued Oct. 17, m 2006 to Hill; and United States Published Patent Applications Nos. 2004/151461, entitled: “Broadband Optical Pump Source for Optical Amplifiers, Planar Optical Amplifiers, Planar Optical Circuits and Planar Optical Lasers Fabricated Using Group IV Semiconductor Nanocrystals”, published Aug. 5, 2004 in the name of Hill; 2004/214,362, entitled: “Doped Semiconductor Nanocrystal Layers and Preparation Thereof”, published Oct. 28, 2004 in the name of Hill et al; and 2004/252,738, entitled: “Light Emitting Diodes and Planar Optical Lasers Using IV Semiconductor Nanocrystals”, published Dec. 16, 2004 in the name of Hill, which are incorporated herein by reference. The aforementioned references relate to different forms of the active semiconductor layer, and to the underlying physical principals of operation of the active semiconductor layers. Accordingly, no serious effort has been made to determine the structural requirements necessary to industrialize or provide practical solutions for manufacturing solid state light emitting devices including the active semiconductor layers.
With reference to FIG. 1, a conventional implementation of a practical light emitting device 1 including the above mentioned materials would consist of a starting conducting substrate 2, e.g. an N+ silicon substrate, on which an active layer 3 of a suitable thickness of dielectric material containing nano-particles would be deposited. The injection of electric current into the active layer 3 and the ability to view any light that might be generated within the active layer 3 will require a transparent conducting electrode to be deposited on top of the active layer 3. Indium Tin Oxide, ITO, is currently the most widely used transparent conducting oxide in opto-electronic devices due to its excellent optical transmission and conductivity characteristics. ITO is a degenerately doped semiconductor with a bandgap of approximately 3.5 eV. Typical sheet resistances measured for the ITO range from as low as 10 Ω/sq to well over 100 Ω/sq . The conductivity is due the very high carrier concentrations found in this material. The work function of the ITO layer 4 is found to be between 4.5 eV and 4.8 eV depending on the deposition conditions. The work function of the N+ silicon substrate 2 is 4.05 eV. The difference in work functions between the ITO layer 4 and the silicon substrate 2 will result in an asymmetry in the electron current injection depending on which interface is biased as the cathode and injecting charge. The work function dominates the contact characteristics and is very important to the stable and reliable operation of any electro-luminescent device.
Subsequently, a metallization step is conducted forming ohmic contacts 5 and 6 onto the ITO layer 4 and the substrate 2, respectively, for injection of electric current. Application of high electric fields will be required for proper operation and the resulting current flow will consist of hot energetic carriers that can damage and change the electronic properties of the optical active layer 3 and any interfaces therewith.
As an example, the substrate 2 is a 0.001 Ω-cm n-type silicon substrate with an approximately 150 nm thick SRSO active layer 3, doped with a rare earth element for optical activity, deposited thereon. The transparent conducting electrode 4 is formed using a 300 nm layer of ITO. Finally metal contact layers 5 are formed using a TiN/Al stack to contact the front side ITO 4, and an Al layer 6 is used to contact the back side of the silicon wafer substrate 2.
At low electric fields in the SRSO active layer 3, there is no current flow and the structure behaves as a capacitor. With the application of an electric field larger than a characteristic threshold field, electrons can be injected into the SRSO active layer 3 from either the N+ substrate 2, via contact 6, or the ITO electrode 4, via contact 5, depending on their bias. Electrons residing in the potential wells due to the silicon nano-particles undergo thermal emission coupled with field induced barrier lowering to tunnel out of the nano-particle traps and into the conduction band of the host SiO2 matrix. Once in the conduction band of the host matrix, the electrons are accelerated by the applied electric field gaining kinetic energy with distance traveled. The distance between the silicon nano-particles will determine the total energy gain of the electrons per hop.
To produce green light at a wavelength of 545 nm, the SRSO active layer 3 may be doped with the rare earth dopant Erbium or Terbium. The energy associated with the emission of a 545 nm photon is approximately 2.3 eV. For current flow between the silicon nano-particles in the active layer 3 to be dominated by ballistic transport, the maximum spacing between the nano-particles should be <5 nm. For a 4 nm spacing, the minimum magnitude of the electric field is found to be approximately 6 MV/cm, at which the conduction electrons can become quite hot and cause considerable damage to the oxide between the nano-particles through the generation of bulk oxide traps and at the interfaces between the silicon substrate 2 and active layer 3, and the active layer 3 and the ITO layer 4 through the creation of interface states. ITO may be susceptible to damage from high electric fields of approximately 1 MV/cm, which is believed may lead to the decomposition of In2O3 and SnO2. If the fields at the surface of the ITO are high enough, the indium and or tin ions can migrate with in the near surface region and concentrate at the active layer interface, this would cause a local reduction in the work function. The work function locally in this region would be reduced to approximately 4.4 eV and 4.2 eV for indium and tin, respectively, which would result in a significant increase in the electron injection characteristics of the ITO layer 4 and the formation of hot spots due to local current hogging potentially leading to device destruction.
The second effect that high electric field have on the device structure is the formation of trapped electronic states located in the band gap of the SiO2 region and interface states located at the active layer/silicon substrate. Generation of trap states in the SiO2 region will reduce the internal electric field and current conduction of the SRSO film requiring the application of higher electric fields to sustain a constant current flow. Positive charge trapping can also occur either through hole injection from the substrate or from impact ionization processes. For conduction electrons with energies >2 eV, traps are formed through the release of hydrogen decorated defects located at the anode. The hydrogen drifts under the applier field towards the cathode where it produces interface states capable of trapping electrons and limiting the current flow.
All of these effects serve to modify, and in some instances increase, the internal electric field in the vicinity of the contact interfaces with the active layer 3, which will lead to an early breakdown and destruction of the light emitting device 1.
Increasing the excess silicon content in the SRSO active layer 3 will cause two things to happen: first, the permittivity of the resulting film will increase due to the presence of the excess silicon, i.e. as the volume concentration of the excess silicon (εsi=11.9 vs εox=3.9) is increased, the permittivity of the silicon will begin to influence and finally dominate the over all permittivity of the SRSO material; and second, the spacing between the nano-particles will be reduced, resulting in a thinning of the barrier presented by the intervening oxide. If this barrier thickness is reduced enough, an increase in direct tunneling between nano-particles will occur. As the excess silicon content of the SRSO active layer 3 is raised, the density of the nano-particles increases and the distance between nano-particles decreases, which enables an increase in the direct overlap of the electron wave function across the thin oxide barrier and the probability of tunneling is increased, i.e. increased conductivity resulting in more current for less electric field. Additionally it is expected that current injection asymmetries due to work function differences between the ITO layer 4 and the N+ silicon substrate 2 will also be reduced. With this increase in direct tunneling, a lower electric field is required to support a given current flow. FIG. 2, illustrates this effect clearly with a plot of refractive index vs electric field strength for active layers with different anneal temperatures, e.g. silicon content. A constant current density of 1.5 mA/cm2 is forced through the active layer 3 and the electric field is determined from the thickness. As can be seen, increasing the excess silicon content as indicated by the increase in index of refraction, results in a significant reduction in the required electric field to sustain a constant current density. This characteristic of large index SRSO active layer films will be used to improve the reliability and hot electron resistance of the optically active SRSO device structure.
An object of the present invention is to overcome the shortcomings of the prior art by the placement of nano-particle rich layers adjacent to the current injecting interfaces to reduce and control the deleterious effects that would result from the hot carriers and their interactions with the operation of this device.
The predominant technologies used today to build solid-state light emitting devices all use various kinds of group III-V or II-VI compound semiconductor materials, such as Aluminum Gallium Indium Phosphide, Indium Gallium Nitride. While such materials have been developed that can provide relatively high internal efficiencies, the high levels of overall power conversion efficiency that are required to be competitive with conventional lighting technologies is proving very difficult to attain. The most significant limitation today is the extraction efficiency, which is a measure of the amount of the internally generated light that leaves the devices to provide useful radiated light. Only with a viable solution to the extraction problem will solid-state technology be able to outperform conventional technologies in efficiency, thus enabling mass adoption. Therefore, any method to improve extraction efficiency is of enormous significance to the solid-state lighting industry.
In a solid-state light emitter, such as an LED, the light is generated within the bulk of the device or in some cases within a thin film. When the light leaves the device to be radiated to the air, rays that are perpendicular to the interface will exit efficiently; however, the rays that reach the interface at an angle greater than the critical angle are subject to total internal reflection and so are not available as useful radiated light and are instead wasted as heat within the device. Wasted light is the primary factor limiting the extraction efficiency in today's solid-state lighting devices. The amount of the loss depends on the amount of mismatch between the refractive index of the emitting material and the refractive index of the external medium, i.e. air in practical cases. For example, with typical LED materials having a refractive index in the range of 2.5 to 4.0, the extraction efficiency to air is only 2% to 4%.
The simplest method commonly used to improve extraction efficiency is to encapsulate the die with a transparent material that has a higher refractive index than air, which reduces the losses due to total internal reflection because the mismatch in the refractive indices is reduced. For example, by using an encapsulant with a refractive index in the range of 1.5 to 1.6, extraction efficiency for conventional LED materials can be raised into the range of 4% to 10%, which is an improvement, but still represents a very low level of efficiency. Therefore, there is a great deal of work being undertaken to find other methods of reducing total internal reflection losses, including surface texturing, silicon lensing, and edge-emission collectors. Many such methods have been described previously, but they all tend to add significant cost and complexity to the manufacturing process, and they typically cannot provide improvements better than a factor of 2. As a result, extraction efficiencies greater than 20% are not practically achievable with any of the materials systems previously envisaged.
The expensive and imperfect mechanisms referenced above attempt to optimize the extraction efficiency despite mismatched refractive indices. In contrast, an object the present invention provides a perfect or near-perfect extraction by building the encapsulant and the light emitting layer from materials having closely matched refractive indices, thus substantially eliminating total internal reflections at the light emitter/encapsulant interface without the need for special surface treatments.
Another object of the present invention is to overcome the shortcomings of the prior art by constructing the emissive area on a single semiconductor substrate in which the shape of the emissive area is defined photo-lithographically, which enables the emissive area to be contiguous or nearly contiguous, and any size, e.g. from cm to meters in length and width, include curved or arcuate lines forming curved geometric shapes, e.g. circles, ovals, ellipsoids. Accordingly, the brightness per unit area can be maximized; any shape and resolution of shapes that can be constructed; and the size of the emissive area can be much more compact, because the whole assembly is constructed monolithically. The light emitted may be of any color, including white. In a variant of this invention, the emissive area may be subdivided into different areas each with its own electrical connection, thus providing an electronic means to vary the beam shape. In a further variant, these different areas may generate light of different colors, so the color of the resulting beam can also be controlled electronically by varying the relative intensity of the different elements. The available color palette may include white and includes control over color temperature and color rendering index.
Moreover, by adopting a process compatible with standard integrated circuits the present invention will be able to integrate complex electronic circuitry on the same chip as the emitting element.